A conventional camera uses a mechanical iris diaphragm to control the amount of light reaching the recording medium, such as film or a charge-coupled device (CCD) light sensor array. The mechanical iris is complex device that is ill-suited for many miniature camera applications, such as those found in smart phones and other hand-held devices. Many electro-optical alternatives to the mechanical iris have been proposed such as the guest-host liquid crystal display (U.S. Pat. No. 4,774,537 and U.S. Patent Application Pub. No. US 2012/0242924), twisted nematic (TN) liquid crystal display (U.S. Patent Application Pub. No. US 2008/0084498), electrophoretic display (U.S. Pat. No. 7,859,741), digital micro-mirror display (European Patent Application No. EP1001619 A1), and electrowetting display (U.S. Pat. No. 7,508,566). All of these alternatives can control the amount of light admitted to the recording medium, and if the electrode structures are patterned in an arrangement of concentric rings to enable changing the aperture, these alternative approaches can also adjust the depth of field. Such a patterned ring arrangement of electrode structures is described in detail, for example, in U.S. Patent Application Pub. No. US 2008/0084498.
Using liquid crystals for an electro-optical iris is particularly attractive since liquid crystal devices (LCDs) are a mature mainstream technology. As with the mechanical iris, to control light effectively, the liquid crystal iris must be capable of providing not only a high contrast ratio, but also a uniform transmittance at intermediate gray levels over the range of light input angles that are typically found in miniature camera optics. Angular dependence of the transmittance of the iris would cause a nonuniform exposure over the light-sensitive area of the recording medium. Prior art liquid crystal iris designs do not satisfy these requirements, which shortcomings are reasons why liquid crystal irises have not yet found widespread commercial application.
Simulations show that a TN device used as a camera iris, as proposed in U.S. Patent Application Pub. No. US 2008/0084498, provides a very high contrast ratio, but the transmittance at intermediate gray levels is appreciably nonuniform over a range of angles of incident light. FIG. 1 shows a simulated electro-optic curve of a prior art TN iris device under conditions of normally incident white light. This curve is simulated by use of Display Modeling System (DIMOS) software available from Autronic-Melchers GmbH, Karlsruhe, Germany, for the liquid crystal mixture MLC-7030 available from Merck GmbH, Darmstadt, Germany. MLC-7030 has a positive dielectric constant anisotropy of 3.8 and a birefringence of Δn=0.1126 at the 550 nm design wavelength. The cell gap is chosen as 4.23 μm to satisfy the “first minimum” condition Δn·d/λ=0.866 to provide maximum throughput at the design wavelength. Using the data of FIG. 1, the normalized transmitted luminance is 50% at 2.81V, and the normalized transmitted luminance is 0.1% at 5.25V, resulting in a contrast ratio of 1,000.
FIG. 2 shows, for this simulation, the angular variation of the normalized transmitted luminance of the prior art TN iris under application of a drive voltage of 2.81V for 50% transmitted luminance at normal incidence. These data are conveniently presented by an iso-transmitted luminance polar contour diagram, in which the contours are lines of constant normalized transmitted luminance. The center of the diagram represents normal incidence, where the normalized transmitted luminance is 50% and the periphery of the diagram represents incident light at the polar angle of 40°. The azimuthal angle of the incident light is represented in the circular direction from 0° to 360°. FIG. 2 shows that the normalized transmitted luminance varies from about 9% to about 82% over incident angles extending out to 20°. Although this TN device can achieve a high contrast ratio, the strong angular variation of intermediate transmittances of this TN device makes it unsatisfactory for use as a camera iris.
The prior art dual-cell guest-host iris represented in FIGS. 6A and 6B of U.S. Patent Application Pub. No. US 2012/0242924, consists of two homogeneously aligned guest-host cells placed in optical series and oriented with their surface alignment directions at 90°. FIG. 3 shows a simulated electro-optic curve for this prior art iris with 5 μm cell gaps filled with the liquid crystal mixture MLC-7030, to which is added an achromatic organic dye mixture with a dichroic ratio of 6.2. Simulations show that, even at 12V, the contrast ratio reaches only 4.1. FIG. 4 shows a simulated iso-transmitted luminance polar contour diagram of the normalized transmitted luminance under application of 3.1V for 50% normalized transmitted luminance at normal incidence. In FIG. 4, the normalized transmitted luminance varies from about 42% to 60% over a range of incident angles extending out to polar angles of 20°. FIG. 4 exhibits somewhat less angular variation of intermediate transmittances compared with the angular variation of the TN iris simulated in FIG. 2, but the low contrast ratio of the dual-cell guest-host iris makes it unsatisfactory for use as a camera iris.